Optical coupling device with a wide bandwidth and reduced power losses

ABSTRACT

A photonic integrated circuit includes an optical coupling device situated between two successive interconnection metal levels. The optical coupling device includes a first optical portion that receives an optical signal having a transverse electric component in a fundamental mode and a transverse magnetic component. A second optical portion converts the transverse magnetic component of the optical signal into a converted transverse electric component in a higher order mode. A third optical portion separates the transverse electric component from the converted transverse electric component and switches the higher order mode to the fundamental mode. A fourth optical portion transmits the transverse electric component to one waveguide and transmits the converted transverse electric component to another waveguide.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. application for patent Ser.No. 15/692,571 filed Aug. 31, 2017, which claims the priority benefit ofFrench Application for Patent No. 1751328, filed on Feb. 20, 2017, thedisclosures of which are hereby incorporated by reference in theirentireties to the maximum extent allowable by law.

TECHNICAL FIELD

Modes of implementation and embodiments relate to integrated photoniccircuits, and in particular to the coupling of input devices, such asoptical fibers, to this type of circuit, and most particularly thecoupling of optical fibers used for the transmission of signals overlong distances.

BACKGROUND

Transceivers based on optical fibers allow the transmission of signalsover long distances. They conventionally use frequency multiplexing soas to be able to transmit/receive several signals with a single opticalfiber. They therefore transmit signals in a wide band of frequencies.

However, today's coupling devices are in general tailored to suit arestricted range of frequencies.

Moreover, an optical signal travelling in a conventional optical fiberis polarized in a random manner, that is to say the orientation of itselectric field is random. Also, an optical signal travelling in aconventional waveguide of an integrated circuit, that is to say awaveguide of rectangular cross section, allows polarization of the lightsignal in two directions only. The first direction, called transverseelectric (TE polarization), is defined as parallel to the layers of theintegrated circuit, for example parallel to the buried insulating layerin technologies of silicon on insulator type. The second direction,called transverse magnetic (TM polarization), is defined as beingorthogonal to the first direction. Certain photonic hardware componentsare particularly suitable for signals polarized in a transverse electricmanner, and other photonic hardware components are particularly suitablefor signals polarized in a transverse magnetic manner. Other hardwarecomponents can receive signals polarized in either way.

Various means exist for coupling an input/output device to an integratedcircuit.

A first solution consists in coupling the input/output device on theupper face of the integrated circuit, and of transmitting the signal tothe waveguide by way of a grating-type coupler.

This solution makes it possible to transmit the transverse electriccomponent of the optical signal as well as the transverse magneticcomponent via its conversion into a transverse electric component.However, this transmission is done only over a very small opticalbandwidth. Consequently a non-negligible part of the optical power,sometimes greater than 50%, may be lost.

A second solution consists in coupling the input/output device on alateral face of the integrated circuit and makes it possible to transmita signal polarized in the transverse electric direction and in thetransverse magnetic direction.

However, the existing solutions are expensive since they require theimplementation of specific methods of fabrication, and they do not allowsufficient confinement of the transverse magnetic component, thus givingrise to diffusion of the signal into the carrier substrate of theintegrated circuit and therefore optical power losses.

It is therefore desirable to limit the diffusion of the transversemagnetic component into the carrier substrate.

SUMMARY

Thus, according to one embodiment, there is proposed a device forcoupling an input device to an integrated circuit allowing thetransmission of a signal of wide bandwidth, whose optical power lossesare reduced, and which is independent of the wavelength of the signal.

Moreover, this coupling device can be produced by conventionalfabrication methods.

According to one aspect, there is proposed a photonic integrated circuitcomprising a substrate surmounted by an interconnection regioncomprising several metal levels, at least two waveguides, and at leastone coupling device situated between two successive metal levels of saidinterconnection region and comprising: a first portion coupled to alateral face of the photonic integrated circuit and configured toreceive an incident optical signal, said signal comprising a transverseelectric component in a fundamental mode and a transverse magneticcomponent, a second portion coupled to the first portion and configuredto convert the transverse magnetic component of the incident signal intoa converted transverse electric component in a higher-order mode, athird portion configured to separate the transverse electric component,here in its fundamental mode, and the converted transverse electriccomponent, here in a higher-order mode, so as to switch the convertedtransverse electric component into a fundamental mode, and a fourthportion configured to transmit the transverse electric component and theconverted transverse electric component to said at least two waveguides.

Thus, by converting the transverse magnetic component, one avoids thelosses that it would have engendered by its diffusion especially in theburied insulating layer of the integrated circuit when the substrate ofthe integrated circuit is a substrate of Silicon On Insulator (SOI)type, while preserving the corresponding power in converted form.

Also, switching the converted transverse electric component into a modedifferent from the fundamental mode makes it possible in particular toprevent the two components of the signal from interfering with oneanother, which would give rise to power losses, or indeed cancellationof the signal in the most unfavorable cases.

According to one embodiment, the first portion comprises a slotwaveguide comprising a first upper band, a first lower band having afirst optical index, and a first intermediate band situated between thefirst upper band and the first lower band and having a second opticalindex lower than the first optical index.

This makes it possible advantageously to confine the transverse magneticcomponent in the first portion of the coupling device, so as to reducethe power losses due to the diffusion of the signal in the buriedinsulating layer.

According to one embodiment, the first portion comprises a first endsituated at the level of said lateral face, and the first lower band andthe first intermediate band have a length greater than the length of thefirst upper band and extend from the first end. The first upper bandextends onwards of a non-zero distance from the first end.

The first portion can have an increasing width and comprise a first endsituated at the level of said lateral face and having a first width, anda second end having a second width, the first width being smaller thanthe second width.

This makes it possible advantageously to tailor the optical indexprogressively in such a way as to limit the optical power losses due,for example, to reflections of the signal.

According to one embodiment, the second portion comprises a polarizationrotator comprising a second upper band, a second lower band and a secondintermediate band, the second upper band and the second intermediateband forming a prolongation of the first upper band and of the firstintermediate band from the second end of the first portion and have adecreasing width so as to attain a smaller width than the second widthat an end of the second portion, and the second lower band forms aprolongation of the first lower band from the second end of the firstportion and has an increasing width so as to attain a third width at theend of the second portion.

The third width can be chosen in such a way that the convertedtransverse electric component is in an optical mode of order one.

According to one embodiment, the third portion comprises a third lowerband and a lateral band which are produced side by side, the third lowerband forming a prolongation of the second lower band and comprises afirst sub-portion of constant width, the lateral band being of constantwidth, the third portion being configured to implement a directionalcoupling of the converted transverse electric component so as to switchthe converted transverse electric component into a fundamental mode.

According to a variant, the third portion can comprise a third lowerband and a lateral band which are produced side by side, the third lowerband forming a prolongation of the second lower band and comprising asecond sub-portion of decreasing width, the lateral band being able tocomprise a third sub-portion of increasing width, the second sub-portionand the third sub-portion being opposite one another, and the thirdportion being configured to implement an adiabatic coupling of theconverted transverse electric component so as to switch the convertedtransverse electric component into a fundamental mode.

Thus, the adiabatic coupling is possible for signals in a wide range offrequencies, thereby making it possible to couple optical signalsindependently of their frequency.

The width of the lateral band can be chosen in such a way that theconverted transverse electric component can travel therein in itsfundamental mode.

The fourth portion can be configured to transmit the transverse electriccomponent to a first waveguide and the converted transverse electriccomponent to a second waveguide.

This makes it possible advantageously to preserve the power of bothcomponents, especially in the case where their combining would generateinterference.

The fourth portion can comprise a first prolongation of the third lowerband parallel to the first waveguide and situated above the latter insuch a way that said first prolongation and the first waveguide aresuitable for the implementation of an adiabatic coupling, and a secondprolongation of the lateral band parallel to the second waveguide andsituated above the latter in such a way that said second prolongationand the second waveguide are suitable for the implementation of anadiabatic coupling.

The intermediate bands can be made of silicon nitride, silicon dioxide,or aluminum nitride, and the other bands of amorphous silicon.

This exhibits the advantage, since amorphous silicon supportstemperatures of up to 500 degrees and since the temperatures used duringthe production of the interconnection part do not exceed 450 degrees, ofallowing the integration of the device into the interconnection part.

Moreover, because of the high optical index of amorphous silicon, thisallows a more significant confinement of the optical signal in thedevice.

BRIEF DESCRIPTION OF THE DRAWINGS

Other advantages and characteristics of the invention will becomeapparent on examining the detailed description of embodiment of theinvention, wholly non-limiting, and the appended drawings in which:

FIG. 1 is a transverse sectional view of a photonic integrated circuit;

FIG. 2 illustrates in a schematic manner a coupling device;

FIGS. 3-5 are sectional views of FIG. 2; and

FIG. 6 illustrates a photonic integrated circuit.

DETAILED DESCRIPTION

FIG. 1 is a transverse sectional view of a photonic integrated circuitaccording to one embodiment.

The integrated circuit comprises a semi-conducting substrate or film 5comprising diverse active hardware components 51 and passive hardwarecomponents 50, which is produced on a buried insulating layer 9 commonlyreferred to by the person skilled in the art by the acronym BOX (“BuriedOXide”), which is itself produced on a carrier substrate (notrepresented).

The semi-conducting substrate 5 is surmounted by an interconnectionregion 6 (or BEOL, “Back End Of Line” according to the acronym wellknown to the person skilled in the art), comprising several metal levels60, 61, 62, 63.

Each metal level comprises one or more metallic tracks 601, encased inan insulating material or Inter-Metal Dielectric (IMD), here silicondioxide. The metal tracks are connected together and to the activehardware components by vias 602 and make it possible to ensure theconnection between the various active hardware components 51 produced inthe substrate 5.

Furthermore, each metal level is delimited at the level of its upper andlower faces by a silicon nitride protection layer, making it possible toavoid the diffusion of the metal of the metallic tracks into the silicondioxide during their production.

Each metal level conventionally has here a height of close to threehundred and fifty nanometers.

The interconnection part 6 comprises on a first level 60 a firstwaveguide GO1 and a second waveguide GO2 (not represented in FIG. 1),produced in a region RG1 of the interconnection part devoid of metal.

The second metal level 61 comprises here a coupling device DIS producedbetween two metal levels 61 and 62, and extending from a lateral face 7of the integrated circuit CI.

The terms “between two metal levels” must be understood here as meaning“situated between the planes containing the lower surfaces of themetallic tracks of two successive metal levels”.

Thus, the device DIS is situated here between the metal levels 61 and62. In a variant, it could equally well have been produced between allother successive metal levels without this affecting the operation.

It should be noted that although for the sake of simplification thefirst waveguide GO1 has been represented as extending perpendicularly tothe plane of FIG. 1 and the device DIS has been represented as extendingparallel to the plane of FIG. 1, in practice these two elements mayextend along parallel directions.

The coupling device DIS is configured to receive an optical signal SIGoriginating for example from an input/output device, for example anoptical fiber, and to transmit it to the waveguides GO1 and GO2.

FIG. 2 illustrates in a schematic manner the coupling device DISaccording to one embodiment.

The device DIS comprises:

-   -   a first portion P1, configured to receive the signal SIG,    -   a second portion P2, configured to convert the transverse        magnetic component of the signal SIG into a converted transverse        electric component,    -   a third portion P3 configured to separate the transverse        electric component and the converted transverse electric        component so as to switch the converted transverse electric        component into a fundamental mode,    -   a fourth portion P4 configured to transmit the transverse        electric component and the converted transverse electric        component to the two waveguides GO1 and GO2.

As illustrated by FIG. 2 and FIG. 3 which is a sectional view of FIG. 2along the sectional axis III-III, the first portion P1 of the device DIScomprises a slot waveguide 1, that is to say a waveguide which comprisesa stack of a first lower band 12, of a first intermediate band 11 and ofa first upper band 10.

The first lower band 12 and the first upper band 10 are here bands ofamorphous silicon each having a thickness of eighty nanometers.

The first intermediate band 11 is here a silicon nitride band having athickness of forty nanometers. As a variant, it would be possible tohave an intermediate band which is made of silicon dioxide or aluminumnitride.

The difference of thickness between the first intermediate band 11 andthe first upper 10 and lower 12 bands, as well as the difference ofoptical index between the amorphous silicon and the silicon nitrideallow the transverse magnetic component of the optical signal SIGtravelling in the slot waveguide 1 to be more confined than if thesignal were travelling in a conventional waveguide.

This makes it possible advantageously to limit the diffusion of theoptical signal towards the buried insulating layer, and therefore tolimit the optical losses.

The configuration of this first portion P1 is given by way ofindication, the thickness of the first upper band 10 and of the firstlower band 12 being able to lie between fifteen nanometers andone-hundred and twenty nanometers, and the thickness of the firstintermediate band 11 being able to lie between twenty nanometers andeighty nanometers.

However, in order to optimize the confinement of the optical signal inthe slot waveguide, it is advantageous to comply with a certain ratiobetween the thickness of the intermediate band layer and that of theupper and lower band layers. The person skilled in the art will know tochoose this ratio as a function of the envisaged applications. Thatsaid, by way of indication this ratio may be of the order of 0.5.

The first lower band 12 comprises here a first end 13 at the level ofthe lateral face 7 of the integrated circuit CI, while the upper band 10and the intermediate band 11 exhibit an offset D with respect to thefirst lower band 12, and do not therefore have an end at the level ofthe lateral face 7.

Here, the offset D is twenty-five micrometers, but could as a variantlie between zero and fifty micrometers.

The presence of an offset makes it possible advantageously to improvethe optical coupling.

In this embodiment, the width of the slot waveguide 1 is increasing.

The waveguide 1 has a first width W1 equal to eighty nanometers at afirst end 13 of the first portion, situated at the level of the lateralface 7 of the integrated circuit, and a second width W2 equal to fivehundred nanometers at a second end 14 of the first portion.

This structure allows better transmission of the optical signal to thecoupling device DIS and therefore a limitation of the optical losses.

By way of indication, the first width W1 can lie between sixtynanometers and two hundred nanometers and the second width W2 can liebetween two hundred nanometers and one thousand five hundred nanometers.

In a particular case, the width of the first portion of the device 2 canbe constant and the first width W1 and the second width W2 then beingequal to two hundred nanometers.

The second portion, of which FIG. 4 is a sectional view along thesection line IV-IV of FIG. 2, comprises a polarization rotator 2,configured to convert the transverse magnetic component of the opticalsignal SIG into a converted transverse electric component.

The rotator 2 comprises a second upper band 20, a second intermediateband 21 and a second lower band 22, which are the prolongationsrespectively of the first upper band 10, of the first intermediate band11 and of the first lower band 12.

Thus, the second end 14 of the first portion and the first end of thesecond portion are merged, and are designated in the figures by the samereference sign 14.

The second upper band 20 and the second intermediate band 21 have adecreasing width, so as to attain a width of eighty nanometers at asecond end 23 of the second portion P2. By way of indication, this widthcould lie between sixty nanometers and two hundred nanometers.

The second lower band 22 has an increasing width, so as to attain athird width W3 at the second end 23 of the second portion P2, forexample here a width of one micrometer. By way of indication, the thirdwidth W3 could lie between two hundred nanometers and one thousand fivehundred nanometers.

Thus, the effective optical index, that is to say the mean optical indexof that region of the device DIS in which the optical signal SIG travels(or stated otherwise, the optical index seen by the signal SIG) variesalong the second portion P2 in such a way that the transverse magneticcomponent of the signal performs a rotation so as to be converted into atransverse electric component.

Thus, the propagation of the transverse magnetic component in theintegrated circuit is avoided, and therefore also the losses engenderedby its diffusion in the buried insulating layer, while preserving thecorresponding optical power since it is propagated in a convertedtransverse electric form.

Here, the dimensions of the second portion P2 are chosen in such a waythat the converted transverse electric component is not in itsfundamental mode, so as not to interfere with the initial transverseelectric component. Here, the converted transverse electric component isin a mode of order 1.

More precisely, to switch a signal from its fundamental mode to ahigher-order mode, it is necessary for the geometry of the waveguidepropagating the signal to vary along the waveguide in such a way thatthe effective index of the fundamental mode and of the higher-order modeof the signal in this waveguide correspond.

The third portion P3 comprises a third lower band 32, which forms aprolongation of the second lower band 22, and a lateral band 30 producedalongside the third lower band, here at a distance of four hundrednanometers.

The third lower band 32 comprises a first sub-portion 33 of constantwidth, and an intermediate sub-portion 36 of decreasing width so as toattain a fourth width W4 of four hundred and fifty-five nanometers. Thisintermediate portion 36 makes it possible to return to a width that ismore appropriate for the propagation of the signal.

The width of the lateral band 30 is constant and equal to the fourthwidth W4.

The first portion 33 and the lateral band are thus configured toimplement a directional coupling.

Here, the length of the first sub-portion 33 is chosen sufficientlyshort for the directional coupling to be able to occur once from thethird lower band 32 to the lateral band 30 but not to be able to occuragain in the reverse direction.

The third portion is thus configured to implement a directionalcoupling, of the converted transverse electric component between thethird lower band 32 and the lateral band 30.

The dimensions of the lateral band are such that the convertedtransverse electric component travels therein in its fundamental mode.

The fourth portion P4, of which FIG. 5 illustrates a sectional viewalong the section line V-V of FIG. 2, comprises the prolongations of thelateral band 30 and of the third lower band 32, which are prolonged intothe fourth portion, forming respectively a first prolongation 41 and asecond prolongation 42, which each extend partially above a distinctwaveguide.

Here, the second prolongation 42 extends in the fourth portion P4 abovethe second waveguide GO2, and the first prolongation 41 extends in thefourth portion P4 above the first waveguide GO1, for example here overlengths of two hundred micrometers.

Thus, it is possible to transmit the transverse electric component tothe first waveguide GO1, and the converted transverse electric componentto the second waveguide GO2, and to use the optical powers in distinctphotonic circuits.

The combining of the transverse electric component and of the convertedtransverse electric component, which could give rise to an optical powerloss because of the interference between the two components, is thuscircumvented.

The vertical distance separating the prolongations 41 and 42 from thetwo waveguides GO1 and GO2 is here two hundred and sixty nanometers, andthe horizontal distance separating the two prolongations is of the orderof ten micrometers.

These two distances are chosen so as to be able to implement anadiabatic coupling of the signal travelling in the prolongations 41 and42 towards the waveguides GO1 and GO2.

Moreover to this end, the prolongations 41 and 42 exhibit decreasingwidths, so as to each have a width of eighty nanometers at theirrespective ends 43 and 44.

Thus, the coupling device makes it possible to minimize the opticallosses and can be produced by conventional methods since its dimensions,here a height of two hundred nanometers, do not involve modifying thedimensions of the other elements of the circuit, in particular thedimensions of the metal levels.

Furthermore, by converting the magnetic component into a transverseelectric component, one circumvents the need to produce hardwarecomponents compatible with each of the components while safeguarding thepower of both components.

FIG. 6 illustrates an embodiment in which the third lower band does notcomprise an intermediate sub-portion but comprises a second sub-portion34 of decreasing width, and the lateral band 30 of the third portion P3comprises a third sub-portion 35 of increasing width, situated oppositethe second sub-portion 34. Here, the third sub-portion 35 varies from aninitial width W5 of eighty nanometers to the fourth width W4.

This advantageously allows the implementation of an adiabatic couplingbetween the third lower band and the lateral band of signals in a widerband of frequencies than in the case of the directional couplingpreviously described in conjunction with FIG. 2.

Indeed, as a function of the frequency of the optical signal, thecoupling will occur earlier or later in the third portion P3, and willonly be able to occur in just one direction.

The coupling device is thus independent of the frequency of the signal.

1. A photonic device, comprising: a first band made of amorphoussemiconductor material and having a length extending from a first end toa second end; an intermediate band made of insulating material; and asecond band made of amorphous semiconductor material and having a lengthextending from a third end to a fourth end; wherein the intermediateband is stacked between and in contact with the first and second bands;said first band having a width, wherein the width of the first bandincreases in a first part of the first band which extends from the firstend to a first intermediate point and wherein the width of the firstband decreases in a second part of the first band which extends from thefirst intermediate point to the second end; and said second band havinga width, wherein the width of the second band increases in a first partof the second band which extends from the third end to a secondintermediate point and wherein the width of the second band furtherincreases in a second part of the second band which extends from thesecond intermediate point to a third intermediate point located adjacentthe second end of the upper band.
 2. The photonic device of claim 1,wherein the first end of the first band is offset in the direction ofsaid length from the third end of the second band.
 3. The photonicdevice of claim 1, wherein the amorphous semiconductor material isamorphous silicon.
 4. The photonic device of claim 1, wherein theinsulating material is selected from the group consisting of: siliconnitride, silicon oxide and aluminum nitride.
 5. The photonic device ofclaim 1, wherein the first parts of the first and second bands form aslot waveguide configured to receive an optical signal.
 6. The photonicdevice of claim 5, wherein the second parts of the first and secondbands form a polarization rotator of the received optical signal.
 7. Thephotonic device of claim 1, wherein the width of the second banddecreases in a third part of the second band which extends from thethird intermediate point to a fourth intermediate point.
 8. The photonicdevice of claim 7, wherein the width of the second band remains constantin a fourth part of the second band which extends from the fourthintermediate point to a fifth intermediate point.
 9. The photonic deviceof claim 8, wherein the width of the second band decreases in a fifthpart of the second band which extends from the fifth intermediate pointto the fourth end, and further comprising an optical waveguidepositioned adjacent the fifth part of the second band.
 10. The photonicdevice of claim 1, further comprising a lateral band made of amorphoussemiconductor material and having a length extending from a fifth end toa sixth end, wherein the lateral band is positioned adjacent the secondband extending from the third intermediate point to the fourth end. 11.The photonic device of claim 10: wherein the width of the second banddecreases in a third part of the second band which extends from thethird intermediate point to a fourth intermediate point; and wherein thewidth of the lateral band is constant in a first part of the lateralband which extends from the fifth end to a fifth intermediate point,said first part of the lateral band located adjacent the third part ofthe second band.
 12. The photonic device of claim 11, wherein the widthof the second band remains constant in a fourth part of the second bandwhich extends from the fourth intermediate point to a sixth intermediatepoint, said first part of the lateral band located adjacent the fourthpart of the second band.
 13. The photonic device of claim 12: whereinthe width of the second band decreases in a fifth part of the secondband which extends from the sixth intermediate point to the fourth end;and wherein the width of the lateral band decreases in a second part ofthe lateral band which extends from the fifth intermediate point to thesixth end; and further comprising: a first optical waveguide positionedadjacent the fifth part of the second band; and a second opticalwaveguide positioned adjacent the second part of the lateral band. 14.The photonic device of claim 10: wherein the width of the second banddecreases in a third part of the second band which extends from thethird intermediate point to a fourth intermediate point; and wherein thewidth of the lateral band increases in a first part of the lateral bandwhich extends from the fifth end to a fifth intermediate point, saidfirst part of the lateral band located adjacent the third part of thesecond band.
 15. The photonic device of claim 14: wherein the width ofthe second band remains constant in a fourth part of the second bandwhich extends from the fourth intermediate point to a sixth intermediatepoint; and wherein the width of the lateral band remains constant in asecond part of the lateral band which extends from the fifthintermediate point to a seventh intermediate point, said second part ofthe lateral band located adjacent the fourth part of the second band.16. The photonic device of claim 15: wherein the width of the secondband decreases in a fifth part of the second band which extends from thesixth intermediate point to the fourth end; and wherein the width of thelateral band decreases in a second part of the lateral band whichextends from the seventh intermediate point to the sixth end; andfurther comprising: a first optical waveguide positioned adjacent thefifth part of the second band; and a second optical waveguide positionedadjacent the second part of the lateral band.